US military commanders were engaged in a desperate hunt for two US soldiers kidnapped by Iraqi insurgents yesterday as thousands of troops massed close to the volatile Sunni city of Ramadi in the largest such crackdown in months.

Helicopters and unmanned aerial drones scoured roads and fields near the town of Yusufiya for evidence of the two soldiers, while divers searched canals and the Euphrates river.

The men went missing after insurgents attacked three US army Humvees at a checkpoint near a canal in Yusufiya on Friday night. The town is a Sunni insurgent hotspot in the so-called Triangle of Death south of Baghdad.

Witnesses said two of the army vehicles gave chase to the attackers, but the third was hit by heavy machine-gun fire before it could move, killing the driver.

Iraqi police sources said yesterday that the two surviving soldiers were then whisked away in two cars by masked gunmen. One of the vehicles was later reported to have been found abandoned with bloodstains in the back.

The New York Times quoted witnesses saying that the soldiers were taken prisoner by a group of seven masked guerrillas. Police said that the area around Yusufiya was virulently anti-American.

US officials say they are asking for the help of local tribal leaders. "We are using all available assets," Major General William Caldwell said. "We never stop looking for our service members until their status is determined, and we continue to pray for their safe return."

But one resident, who said his house was searched yesterday, claimed that he had refused to cooperate despite being offered a reward of $100,000 (£53,000).

The manhunt occurred as the US military staged one of its largest operations for months in an attempt to disrupt militant supply lines from the Sunni Arab city of Ramadi, west of Baghdad.

At dawn yesterday US and Iraqi troops established a number of outposts on the southern side of the city while US and Iraqi tanks and Iraqi sniper teams patrolled the streets and Marine boats scanned rivers and canals. US commanders said the new outposts would be manned by Iraqi forces and would conduct patrols and monitor movements along major roads into the city.

With a population of some 400,000 mostly Sunni Arabs, the capital of Anbar province has grown increasingly lawless. Insurgents, many of whom arrived following the US assault on nearby Falluja, are said to control large areas of the city, using their presence to channel funds and weapons to militants across the Sunni Triangle.

Earlier this month, an extra 1,500 US troops were brought into Iraq to help wrest control of Ramadi, fuelling residents' fears of a large-scale assault. But the US military said yesterday there was no reason to panic and denied they were planning a Falluja-style offensive.

"We are focusing on multiple sites used by the insurgents to plan and conduct terrorist attacks and store weapons," army spokesman Lieutenant Colonel Bryan Salas told Reuters. "We have set up checkpoints to restrict the flow of insurgents, but citizens will still be able to enter and leave the city."

The weekend also saw a string of bloody bomb and gun attacks that killed at least 47 people in and around Baghdad in defiance of the security crackdown. Fears for the safety of the kidnapped US soldiers have been heightened because Iraq's Sunni Arab community is already angered by the reports of the alleged murder of 24 civilians by US troops in Haditha last November. Sunni Arabs also complain of arrests and arbitrary executions at the hands of the country's Shia-dominated security forces and militias.

The prime minister, Nuri al-Maliki, a Shia, had vowed to reach out to the disaffected Sunni Arabs, who form the backbone of the insurgency. He has announced a prisoner release programme affecting mostly Sunni detainees, and this week he is expected to unveil a "plan for national reconciliation" to include amnesties for "low-level insurgents".